Inkjet printing and energy-curable inks have experienced significant development over the last decade. In general, these developments have focused on more effective and efficient means to cure the ink after it has been deposited onto a substrate.
The first energy-curable inkjet printing systems used medium pressure Mercury (vapor) bulbs. These bulbs were capable of producing a significant peak intensity (W/cm2) and doses of UV radiation (J/cm2) in a variety of wavelengths. UV radiation is categorized based on the emitted wavelength. Traditionally, there were three recognized categories: electromagnetic radiation subtype A (UVA) (400 to 315 nanometers), electromagnetic radiation subtype B (UVB) (315 to 280 nm), and electromagnetic radiation subtype C (UVC) (280 to 100 nm). Photoinitiators distributed throughout the ink are able to capture the UV photons emitted by the bulbs. The photoinitiators decomposed into free radicals when exposed to light, which promoted cross-linking at the surface and within the bulk of the ink.
Improvements were made to the medium pressure mercury bulbs by doping the bulbs with small amounts of iron, gallium, etc. These metals changed the distribution of the UV wavelengths emitted by the bulbs. For example, doping using iron caused the emission spectrum to shift higher, i.e. higher wavelength. Higher wavelengths can be beneficial for improving the depth at which curing takes place.
Although medium pressure mercury bulbs have been widely used, they are not without significant drawbacks. The bulbs tend to operate at a very high temperature (bulb surface can reach 650-900° C.), which then imparts heat to the substrate. These temperatures can cause substantial problems if the substrate is thin or heat-sensitive. Furthermore, the amount of UV emitted by the bulb is correlated with the heat of the bulb. Accordingly, if a given substrate requires that the bulb be turned down, i.e. lower intensity/temperature, then the bulb's ability to effectively cure is affected. This can result in poor adhesion, surface tackiness, etc. Various technologies have been used in an effort to reduce the temperature emitted by the bulbs, including dichroic reflectors and air and/or water cooling systems.
Advancements in UV light emitting diode (LED) lamp technologies have overcome some of the shortcomings associated with medium pressure mercury bulbs. Although widely-available LED lamps have a relatively limited wavelength range, e.g. 405 nm, 395 nm, 385 nm, 365 nm, the lamps exhibit a high peak intensity (16+W/cm2). UV LED lamps are often used in conjunction with special ink formulations, which result in much lower heat output (and a wider range of potential substrates). UV LED lamps are also associated with lower power consumption and much longer lifetimes with more predictable power output.
However, at these wavelength ranges, i.e. 365 nm to 405 nm, limited curing occurs at the surface of the ink. In general, the curing is limited by oxygen radicals present at the ink's surface. Oxygen rapidly diffuses into the ink when a drop is ejected from the printer head and spreads out after impact with the surface of the substrate. The oxygen radicals found near the surface of the ink inhibit network formation and cross-linking.
Prior technologies have focused on how to reduce and/or eliminate oxygen present near the surface of the ink. One alternative is to use a nitrogen “blanket” that is created using compressed air and a filter that separates nitrogen and oxygen from the compressed air. Nitrogen concentrations of above 99% are possible. The filter pumps the filtered air over the surface of the ink, thereby reducing or eliminating the presence of oxygen. However, adding a suitable onboard filter and compressed air supply can prove difficult. For example, a smaller printer may not have access to compressed air, while a larger printer may require a large amount of Nitrogen, e.g. upwards of 200 L/min. These limitations may be prohibitive (cost, space, etc.) for many printer installations.
A second alternative is to modify the composition of the ink. More specifically, there are a number of chemical compositions that may be used to increase the surface cure of the ink, even in the presence of oxygen. The most effective chemical composition used today is N-vinyl caprolactam (V-Cap). Despite its effectiveness in promoting effective curing, the hazard classification for V-Cap has recently been modified, in particular for those ink formulations in which the V-Cap concentration exceeds 1% or 10%. Historically, V-Cap concentrations of more than 40% were used by some ink manufacturers. Thus, many ink manufacturers have begun searching for alternative means to facilitate surface curing.